At present, various methods are used to coat and treat paper and paperboard. Coating is often carried out in two phases so that first, the coating is spread onto the web and the final layer of coating is made separately by using a coating knife, a rod or an airbrush. Treating methods are also available, which provide the correct and sufficiently even amount of treating agent without doctoring. Such methods include, for example, spray coating and, in some cases, jet or curtain coating. In short dwell time coating, the layer of coating or size is immediately evened in connection with the application by using the coating knife or rod that defines the application chamber. The manner of spreading and evening the treating agent has an effect on the product's properties and, thus, the selection of the method of treatment can provide end product qualities suitable for various purposes.
Most spreading devices for treating agents that are currently in use are large and complex. All the devices have limitations in operation related to the product's quality, production rate, and other matters, which is why they are usually suitable for producing a limited product range only. Fitting bulky equipment in place of old equipment when renewing machines is difficult, limiting the alternatives used in modernization. One problem with the applicators most frequently used is that the devices use a large overflow and number of applications, so that part of the coating is returned, as return flow, either from the applicator or the doctor to a tank, from where it is again pumped into the applicator. In that case, the treating agent gets in contact with air and forms air bubbles, which must be removed during the circulation of the treating agent before the agent can be fed into the applicator again. Air removal is difficult and deaeration devices are expensive. In return circulation, the treating agent can easily get dirty or otherwise contaminated, and we must ensure that the agent remains clean. Normally, biocides must also be added to the treating agent to prevent the growth of bacteria, which causes various problems. Consequently, there is a need for new methods of treating paper and board webs. In paperboard manufacture in particular, it would be preferable to provide such a method and a device to spread surface size on the surface of the web, which makes it possible to apply a sufficient amount of size on the web's surface at a considerably higher velocity than before, but which would be cost-effective enough also when used in board machines with low production volumes. Adjusting the amount and the profile of size in production is also an especially big problem.